Properties of S-adenosyl-L-methionine-magnesium-protoporphyrin IX methyltransferase from barley

S-Adenosyl-L-methionine-magnesium-protoporphyrin IX methyltransferase (EC 2.1.1.11) is present in greening barley seedlings associated with the particulate fraction. This enzyme was purified 20 fold using protamine and ammonium sulfate precipitation. The enzyme was active over a wide pH range with highest activity at pH 7.5. The Km values for Mg-protoporphyrin IX and S-adenosylmethionine were 48 and 39 μM, respectively; S-adenosylethionine and S-adenosyihomocysteine were competitive inhibitors with respect to S-adenosylmethionine; hemin inhibition was non-competitive with respect to Mg-protoporphyrin IX; thiol compounds exhibited a stimulatory effect on enzyme activity. The properties of the enzyme are discussed and compared with the enzyme from other organisms.